COCCI Profile: Sensitivity Training

European study backs earlier work in US showing that coccidiosis vaccination restores Eimeria sensitivity.

Landman and Peek: 'Resistance is a problem of increasing importance.'

Coccidiosis vaccination appears to restore the sensitivity of European coccidial field isolates to commercial in-feed anticoccidials, according to the results of a study conducted by a leading animal health institute.

"For poultry producers, restoration of sensitivity would enable better, more efficient use of anticoccidial drugs and improved coccidiosis control," says Dr. W.J. Landman, an investigator in the study, conducted by the Animal Health Service Ltd. Poultry Health Centre, the Netherlands, one of the largest and best equipped animal diagnostic laboratories in the world.

In-feed anticoccidials have long been the primary method of controlling Eimeria, the protozoan coccidial parasite that causes coccidiosis in poultry, but widespread resistance to the drugs has developed after continuous usage, say Landman and his associate in the study, H.W. Peek.

The investigators have researched the sensitivity of coccidia in poultry since the late 1980s. In 2003, they published the results of an extensive survey with coccidial isolates harvested in the Netherlands from 1996 to 2001 and reported extensive resistance to commercial anticoccidials. A paper published in 2004 showed similar resultsfor isolates taken from Spain and Germany. Other researchers have also documented anticoccidial resistance, Landman says.

Resistance impedes efficient coccidiosis control

"Resistance is a problem of increasing importance that is impeding the efficient prevention of coccidiosis," Landman and Peek say.

Because anticoccidial resistance can affect flocks gradually, some poultry producers are not aware of the problem or that flock performance is not as good as it could be; others live with the reduced performance caused by coccidiosis, Landman says. "Sometimes, the resistance is not absolute and the infection is tempered, but it's there," he adds.

Table 1. Sensitivity findings of three Eimeria species to diclazuril and monensin.

In the United States, it is well documented that coccidial sensitivity to anticoccidial drugs can be restored with the use of coccidiosis vaccination, which replaces wild field strains with drug-sensitive strains. In Europe, however, the recent study by Landman and Peek is believed to be the first of its kind to show that vaccination can restore anticoccidial sensitivity in the field.

In their most recent study, they determined anticoccidial drug sensitivity for 21 Eimeria field isolates originating from broiler farms in Denmark, Germany, Great Britain, Greece, Italy, Portugal and Romania. The isolates were supplied by Schering-Plough Animal Health Corporation. The in-feed anticoccidials tested were diclazuril (Clinacox) and monensin (Elancoban).

There were four groups of nine chicks that each received the following:

Group 1: Infected, received diclazuril at a dose of 1 mg/kg in feed
Group 2: Infected, received monensin at a dose of 100 mg/kg in feed
Group 3: Infected but not treated with anticoccidials (positive control)
Group 4: Not infected and not treated (negative control)

At 8 days of age, the investigators exposed the first three groups to a defined number of sporulated oocysts (oocysts capable of releasing viable spores) prepared from the Eimeria field isolates. The isolates in the study were E. acervulina, E. tenella and E. maxima, which are all known to cause clinical coccidiosis in chickens.

The two treated groups started on medicated feed 2 days before the challenge and continued until the end of the experiment. The researchers conducted a chemical analysis of the anticoccidial products in the feed to assure that product concentrations were close to the desired dose.

At 14 days of age, or 6 days after the challenge, they necropsied five birds from Groups 1, 2 and 3 to determine individual coccidial lesion scores. They also counted per gram the number of oocysts in fresh feces, weighed birds periodically and observed them throughout the study.

Anticoccidial sensitivity profile

Table 2. More Eimeria isolates were sensitive to anticoccidials that were from vaccinated flocks.

The anticoccidial sensitivity profile for each Eimeria isolate was based on the percentage reduction of the median lesion score for each Eimeria type as compared to the infected, treated group (Group 3). A reduction of 0% to 30% indicated coccidial resistance, 31% to 49% indicated reduced sensitivity or partial resistance and 50% or more indicated full sensitivity to the anticoccidial, the investigators say.

There was significant resistance to both anticoccidials (Table 1), especially E. acervulina. Of these isolates, 70% were resistance to diclazuril and 55% were resistance to monensin, say Landman and Peek.

"The findings of this study show that resistance is still widespread among European coccidiosis field isolates," the investigators conclude.

Role of vaccination

The investigators also discovered that vaccination appeared to affect sensitivity. "Furthermore, our findings suggest that sensitivity toward both anticoccidial drugs tested occurs more frequently in Eimeria spp. field isolates originating from broiler farms where a coccidiosis vaccination policy is followed," they say.

Table 2 summarizes the Eimeria species that were sensitive to diclazuril or monensin and the number of them that came from farms that had or had not vaccinated.

Landman and Peek are further studying the effects of vaccination on Eimeria sensitivity with larger numbers of birds. It is always possible that the findings might change, says Landman, but the results of the just completed study "very strongly suggest" that vaccination does, indeed, restore Eimeria sensitivity to both anticoccidials tested.

Source: CocciForum Issue No.10, Schering-Plough Animal Health.